Optical processing in asynchronous transfer mode network

ABSTRACT

An optical code recognition unit (OCRU) for recognizing a predetermined n-bit optical code has an n-way splitter having an input and n parallel outputs. A plurality of combiners are associated with the splitter outputs, and a respective gate is controlled by the output of each of the combiners. Each of the splitter outputs is subject to a different delay of from 0 to (n-1) bit periods, and each combiner receives an input from at least one of the splitter outputs. The OCRU is such that all the gates are turned on if a predetermined optical code is applied to the splitter input.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical processing in networks carryingpacketised signals, and in particular to an all-optical code recognitionunit for such a network.

2. Related Art

Optical fibre communication offers many advantages over conventionalwire based systems, these advantages including reduced losses, increasedbandwidth, immunity from electromagnetic interference (EMI), and a highlevel of security. The application of optical fibre technology into thelocal area network (LAN) is, therefore, of increasing interest. In thepast, however, it has been assumed that optical networks will onlypenetrate small business and residential sectors if new broadbandservices are provided to offset the additional costs involved in theinstallation of the optical technology. Some of the broadband servicesthat could be provided are alpha-numeric videotex (e.g. Prestel),photographic videotex, high definition television, interactive video ondemand (video library), video telephony, interactive graphics andhigh-speed data services.

Although the importance of providing such services has been recognisedfor some time, it is difficult for telecommunications operatingcompanies to predict their market potential and therefore justify amajor investment. What is required is an entry strategy that allowsoptical technology to be installed economically for telephony andlow-speed data services, while maintaining the potential for evolutionat a marginal cost for future broadband services.

In known optical networks, routing of information is achieved at eachnode by electronic means, that is to say by detecting the receivedoptical signal to give an electrical signal (plus detector noise). Thiselectrical signal must be regenerated, after processing and switching toremove the noise, before the signal is re-transmitted optically.Regeneration is bit-rate dependent, and so restricts the informationtype that can be carried, thereby preventing the transmission ofbroadband services. The need for regeneration could be removed bycoupling off, at each node, part of the received optical signal, thecoupled-off signal being converted to an electronic signal which iselectronically processed, the remaining uncoupled optical signal beingrerouted by the electronic processor. Unfortunately, the electronicprocessing times severely limit the possible capacity of the opticallinks, so again the provision of broadband services is not practical.Thus, although the electronic processor can switch quickly (of the orderof nanoseconds) it requires a relatively long time (of the order ofmicroseconds) to process, and therefore to decide upon the necessaryroute of the signal. In this scheme, the uncoupled optical signal isdelayed during the processing time by a long length of optical fibre,and this obviously increases the size of each switching node.

Optical routing of information at the nodes of such an optical networkwould increase the capacity of the network by reducing the processingtime. Not only would this increase the capacity of the network, it wouldalso decrease the vast delay lengths of optical fibre otherwiserequired. Optical signal processing is well known, but the particularmethod of optical routing in a given network will depend upon the natureof that network. A particularly advantageous type of optical network isthe recently developed telephony passive optical network (TPON). Thistype of network has no routing mechanisms, that is to say all terminalsreceive all the information transmitted by the exchange. One of the mainadvantages offered by TPON is the ability to move transmission betweencustomers. This is because the gross bit-rate used with TPON is 20Mbit/s (chosen mainly to allow cheap CMOS realisation of signalprocessing chips), and this is divided into a basic granularity of 8Kbit/s, that is to say 8 Kbit/s is the basic transmission unit that canbe moved from customer to customer (or can be summed to provide channelsof n×8 Kbit/s capacity). This ability suggests that TPON will beparticularly applicable to the small business sector. TPON also showsgreat promise for the economic provision of optical fibre to thetelephony customer, with major potential for later extension tobroadband integrated services digital networks (ISDN).

In order to enhance management and flexibility of the core of thenetwork of the telecommunications network, a synchronous digitalhierarchy (SDH) managed transmission network is planned forimplementation by BT from 1992 as a replacement for the presentasynchronous trunk and junction networks. An SDH network would have fourdifferent levels, with a passive optical network (PON) at the lowest(Access) level, and a high capacity routed network at the upper (InnerCore or Long Haul) level. The Inner Core level would benefit the mostfrom optically-controlled routing, as this level requires the largestcapacity. The increase in capacity required at the Access level (becauseof the addition of extra services) would, however, benefit from the useof optical routing. At the Access level it is envisaged that there wouldbe sixty-four access points to each node. It would, therefore, bepossible to address each individual node by a series of code sequences,each code sequence allowing up to sixty-four permutations.

One method of implementing an SDH network, that achieves flexibility andsupports the divergent needs of future services, is based on packetswitching which is currently used in data networks where error-freecommunication is required. The protocols required for such a systemcontain sophisticated methods for correcting, retransmitting orre-routing packets, and so need a lot of processing which can cause longdelays. To accommodate delay-critical, but error-tolerant services, suchas voice, a much simpler protocol can be used to minimise the processingtime required. An example of this technique, which is known asasynchronous transfer mode (ATM) is used for fast packet switching orasynchronous time division (ATD).

ATM is a label multiplexing technique that uses short, fixed lengthpackets, or cells. The ATM cells are short to reduce packetisationdelay, and are of fixed length to make it easier to bound delays throughswitches and multiplexers. They have short labels (or headers) to allowcells to be routed, at high speeds, by means of hardware routing tablesat each switch. For large transmission bandwidths (-1 Gbit/s or more)this routing may be most effectively performed optically via opticalcode recognition (OCR).

The packet header and information fields must be separated at nodeswhere OCR of the header is to take place. This could be achieved byhaving the information field at bit-rates far in excess of the headerbit rate and the response time of the optical code recognition unit(OCRU), so that the OCRU, being too slow to "see" the information fieldbit rate will only "see" a constant intensity after the header.Alternatively, and preferably, the header and information fields couldbe at different wavelengths, so that they may be split easily, either bya wavelength dependent coupler or by means of wavelength divisionmultiplexing technology.

In developing a system of optical code recognition for use in opticalrouting of a TPON, the following requirements must be met, namely:

(a) Around 64 codes are required with the minimum of redundancy. This isdue to the SDH network requiring up to 64 codes at each level of thenetwork adequately to address each access terminal;

(b) The CCRU should be self timing, that is to say a clock signal shouldnot be required to synchronise the OCRU;

(c) The OCRU should be realised using the minimum number of components,thus keeping cost and complexity down;

(d) The match/mismatch decision of the OCRU must be achieved veryquickly (that is to say the OCRU must have lower processing times thanelectronic systems;) and

(e) The logic level of the OCRU output should be kept to a minimum,since multiple level logic is easily degraded by the noise that isalways present in real systems.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an OCRU for recognising a predeterminedn-bit optical code, the OCRU comprising an n-way splitter having aninput and n parallel outputs, a plurality of combiners associated withthe splitter outputs, and a respective gaze controlled by the output ofeach of the combiners, wherein each of the splitter outputs is subjectto a different delay of from 0 to (n-1) bit periods, and each combinerreceives an input from at least one of the splitter outputs, and whereinthe OCRU is such that all the gates are turned on if a predeterminedoptical code is applied to the splitter input.

Advantageously, each combiner is configured to operate at 2-level logic,and the arrangement is such that, when the predetermined optical code isinput to the n-way splitter, each combiner receives an input of one ormore `0`s or one or more `1`s, and each combiner receiving `1` inputsreceives a maximum of two such inputs.

Preferably, the gates are positioned in series between an input deviceand an output device, whereby a signal input by the input device willreach the output device if the predetermined code is input to the n-waysplitter. Each of the gates may be constituted by a semiconductor laseramplifier (SLA).

The invention also provides a system for processing packetised signalsin a network comprising a head-end packet signal transmitter and aplurality of customer receivers, the system comprising a respectiveapparatus associated with each customer receiver, each apparatuscomprising separator means for separating header field information fromdata field information in packets, first transmission means fortransmitting the header field information to a switch associated withthe respective customer receiver, and second transmission means fortransmitting the data field information to said switch, wherein eachfirst transmission means includes an OCRU as defined above, and whereineach apparatus is such that the respective OCRU activates the associatedswitch to permit the passage of the header field information of a givenpacket only if the optical code contained in the header fieldinformation of that packet is the predetermined optical code of thatOCRU.

Advantageously, each of said switches is a bistable switch such as anSLA.

Conveniently, a respective wavelength-dependent coupler constitutes theseparator means of each apparatus.

Preferably, the second transmission means of each apparatus includes anoptical delay fibre of such a length that the header field informationof a given packet reaches the switch substantially as the switch isactivated by the OCRU.

In a preferred embodiment, the network is a packet switched network, thehead-end packet signal transmitter is a head-end packet transmitter, andthe packets are cells consisting of headers and data.

BRIEF DESCRIPTION OF THE DRAWINGS

An optical routing apparatus incorporating an optical code recognitionunit constructed in accordance with the invention will now be describedin greater detail, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic representation of the routing apparatus;

FIG. 2 is a schematic representation of an optical code recognition unitforming part of the apparatus of FIG. 1; and

FIG. 3 is a schematic representation of part of a modified optical coderecognition unit.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to the drawings, FIG. 1 is a schematic representation of acustomer-end optical routing apparatus for use with a TPON systemcarrying packetised signals (one cell of which is indicated by thereference numeral 1). Each cell 1 has a data field 1a and a header field1b, these two fields being transmitted at different wavelengths. Thecustomer-end routing apparatus includes a wavelength dependent coupler 2which separates the header field information from the data fieldinformation. The header field information is fed to a bistable switch 3(and then on to the customer's receiver 4) via an OCRU 5. The data fieldinformation is fed to the bistable switch 3 via a delay fibre 6. Thebistable switch is constituted by a split-contact laser amplifier havinga maximum rise time of less than 200 psec.

The OCRU 5 is configured to a particular optical code which is unique tothe customer concerned, the optical code corresponding to all or part ofthe header field 1b. The OCRU 5 will, therefore, provide an outputsignal only when recognises the particular optical code appropriate tothe customer. This output signal is used to control the bistable switch3 so that the data field information is routed to the receiver 4. Thedelay fibre 6 is chosen to ensure that the data field 1a of the samecell 1 as the header field 1b recognised by the OCRU 5 is passed to thereceiver 4. Consequently, signals (packets) intended for other customerswill not be routed to that particular customer's receiver 4.

FIG. 2 shows the OCKU 5, this OCRU being configured to recognise theoptical code 0010111. The OCRU 5 includes a passive seven-way opticalsplitter 7 having seven parallel output fibres 7a, three opticalcombiners 8a, 8b and 8c and three SLA gates 9a, 9b and 9c. Each of thefibres 7a is given a different delay of from 0 to 6 bit periods (asindicated FIG. 2). The effect of the splitter 7 is, therefore, toconvert the serial input code into a parallel output code, with one bitof the code on each of the output lines 7a.

In the particular OCRU 5 shown in FIG. 2, a first pair of output lines7a which should carry `1`s are input into the optical combiner 8a, asecond pair of output lines 7a which should carry `1`s are input intothe optical combiner 8b, and the remaining three output lines 7a (whichshould carry `0`s) are input into the optical combiner 8c. The combiners8a, 8b and 8c are SLAs which operate under gain saturation. If thecombiner 8a does receive two `1`s at its input, its output will be atthe `2` intensity level. Similarly, if the combiner 8b receives two`1`s, it will output a `2`, and the combiner 8c will output a `0` forthree input `0`s.

The SLA gate 9a, which receives the output of the combiner 8a, isconfigured to switch on for a `2` level input. Similarly, the gate 9bwill switch on if it receives a `2` level input from the combiner 8c.Consequently, if the OCKU 5 does receive the `correct` code 0010111, allthree gates 9a, 9b and 9c will be turned on, and an input signal 10 froma continuous wave (cw) laser (not shown) will be passed to the bistableswitch 3. The switch 3 will then be turned on, so that the informationcarried by the data field 1a of that cell whose header field 1b carriesthat code is passed to the associated receiver 4. It will be appreciatedthat a match of the code will he recognised almost instantaneously withthe input of the final bit of the code, so that the processing time ofthe OCRU 5 is almost zero. As the combiners 8a, 8b and 8c are configuredto operate at 2-level logic (that is to say they each have a threeintensity level output `0`, `1`, `2`), the entire OCRU 5 operates at2-level logic. This avoidance of multiple-level logic is advantageous,in that multiple-level logic is easily degraded by the noise that isalways present in real systems.

FIG. 3 shows the gating arrangement of an alternative form of OCRU 5',this OCRU being configured to recognise the optical code 1010111. As theOCRU 5' is required to recognise a code having an odd number of `1`s, anadditional (and differently-configured) SLA gate is needed. Thus, theOCRU 5' has four gates 9a', 9b', 9c' and 9d', the gates 9a', 9b' and 9d'being identical with the gates 9a, 9b and 9c of the OCRU 5 of FIG. 2.The gate 9c' is configured to switch on for a `1` level input from itscombiner (not shown), this combiner being configured to output a `1` ifits input receives a `1`.

Clearly, the particular form of OCRU required for each customer willdepend upon the code allocated to that customer. In particular, thenumbers and configurations of combiners and gates will depend upon thenumber of `1`s in the code to be recognised. In each case, however, theOCRU will operate at 2-level logic, and maximum number of SLA gates willbe four for a 7-bit code.

One disadvantage of the OCRU described above is that the bistable switch3 outputs only the data field 1a of the recognised cell. An additionaldevice such as an optical transmitter must, therefore, be provided tore-input the header field 1b for each such recognised cell. To removethe need for this additional device, the OCRU may be modified byreplacing the coupler 2 with a 90/10 splitter, 90% the signal beingdirected towards the bistable switch 3, and 10% towards the OCRU. Inthis case, the header field 1b is distinguished from the data field 1ain the OCRU by its modulation speed (the modulation speed of the datafield being too fast for the response time of the gates). When a headerfield 1b is recognised by the OCRU, the bistable switch 3 is triggeredto pass the 90% part of the signal, so that header information is passedalong with the data.

In a further alternative, a time-dependent switch can be used toseparate the header field 1b from the data field 1a. This switch wouldbe triggered by a clock signal extracted from the main input signal.

It will be apparent that the routing apparatus of the invention couldhandle any form of packetised signal, where the packets (or cells) aredivided into header byte(s) and data byte(s), such as the ATM format.Although at the current agreed maximum rate of 140 Mbit/s opticalrouting is unlikely to be beneficial, standard agreement at higher ratescould change this situation.

We claim:
 1. An OCRU for recognising a predetermined n-bit optical code,the OCRU comprising:an n-way passive optical splitter having an inputand n parallel outputs, a plurality of combiners optically connected tothe splitter outputs, and a respective gate controlled by the output ofeach of the combiners, wherein each of the splitter outputs is subjectto a different delay of from 0 to (n-1) bit periods, and each combinerreceives an input from at least one of the splitter outputs, and whereinthe OCRU gates are turned on if a predetermined optical code is appliedto the splitter input.
 2. An OCRU as in claim 1, wherein each combineris configured to operate at 2-level logic.
 3. An OCRU as in claim 1,wherein each of the gates includes an SLA.
 4. An OCRU for recognising apredetermined n-bit optical code, the OCRU comprising:an n-way splitterhaving an input and n parallel outputs, a plurality of combinersoptically connected to the splitter outputs, and a respective gatecontrolled by the output of each of the combiners, wherein each of thesplitter outputs is subject to a different delay of from 0 to (n-1) bitperiods, and each combiner receives an input from at least one of thesplitter outputs, wherein the OCRU is such that all the gates are turnedon if a predetermined optical code is applied to the splitter input, andwherein, when the predetermined optical code is input to the n-waysplitter, each combiner receives an input of one or more `0`s or one ormore `1`s, and each combiner receiving `1` inputs receives a maximum oftwo such inputs.
 5. An OCRU for recognising a predetermined n-bitoptical code, the OCRU comprising:an n-way splitter having an input andn parallel outputs, a plurality of combiners optically connected to thesplitter outputs, and a respective gate controlled by the output of eachof the combiners, wherein each of the splitter outputs is subject to adifferent delay of from 0 to (n-1) bit periods, and each combinerreceives an input from at least one of the splitter outputs, wherein theOCRU is such that all the gates are turned on if a predetermined opticalcode is applied to the splitter input, and wherein the gates arepositioned in series between an input device and an output device,whereby a signal input by the input device will reach the output deviceif the predetermined code is input to the n-way splitter.
 6. A systemfor processing packetised signals in a network comprising a head-endpacket signal transmitter and an plurality of customer receivers, thesystem comprising:a respective apparatus associated with each customerreceiver, each apparatus comprising separator means for separatingheader field information from data field information in packets, firsttransmission means for transmitting the header field information to aswitch associated with the respective customer receiver, and secondtransmission means for transmitting the data field information to saidswitch, wherein each first transmission means includes an OCRU forrecognising a predetermined n-bit optical code, the OCRU comprising: ann-way splitter having an input and n parallel outputs, a plurality ofcombiners optically connected to the splitter outputs, and a respectivegate controlled by the output of each of the combiners, wherein each ofthe splitter outputs is subject to a different delay of from 0 to (n-1)bit periods, and each combiner receives an input from at least one ofthe splitter outputs, wherein the OCRU is such that all the gates areturned on if a predetermined optical code is applied to the splitterinput, and wherein each respective OCRU activates its respective switchto permit the passage of the header field information of a given packetonly if the optical code contained in the header field information ofthat packet is the predetermined optical code of that OCRU.
 7. A systemas in claim 6, wherein each of said switches is a bistable switch.
 8. Asystem as in claim 7, wherein each of the bistable switches includes anSLA.
 9. A system as in claim 6, wherein a respectivewavelength-dependent coupler is included in the separator means of eachapparatus.
 10. A system as in claim 6, wherein the second transmissionmeans of each apparatus includes an optical delay fibre of such a lengththat the header field information of a given packet reaches the switchsubstantially as the switch is activated by the OCRU.
 11. A system as inclaim 6, wherein the network is a packet switched network, the head-endpacket signal transmitter is a head-end packet transmitter, and thepackets are cells including headers and data.
 12. An OCRU forrecognising a predetermined n-bit optical code, said OCRU comprising:ann-way passive optical splitter having a bit-serial optical input and ann-bit parallel optical output; and an optical signal logic networkoptically coupled to the n-way passive optical splitter outputs and tocontrol the state of a bi-stable optical gate in response to theoccurrence of a predetermined n-bit optical code input to the OCRU.